The present invention generally relates to a semiconductor integrated circuit device having a digital signal processing circuit and an analog signal processing circuit which are provided on a single chip.
A one-chip microcomputer is widely used. Generally, a one-chip microcomputer includes a central processing unit, a random access memory, a read only memory, a timing generator and an input/output port. A more advanced one-chip microcomputer further includes an analog signal processing circuit, such as an analog-to-digital converter (hereinafter simply referred to as an A/D converter) in order to provide an improved processing ability. The A/D converter built in the one-chip microcomputer receives an analog input signal from an external device and converts the same into a digital signal. Currently, a high-precision A/D converter is required. Conventionally, a digital signal processing circuit, such as a CPU, and an analog signal processing circuit, such as an A/D converter, are driven by a system clock or respective clocks which are generated from the system clock.
It will be noted that the arrangement of interconnection (wiring) lines on the chip becomes more complex with an increase in the integration density. Thus, there is a possibility of clock signal lines and power supply lines being close to each other. In this case, the clock signal lines and the power supply lines are electrostatically coupled to each other so that coupling capacitance will be formed therebetween. The power supply lines are affected by a level change of the system clock signal from a high level to a low level and vice versa so that noise components are superimposed on power supply voltages. Such noise components superimposed on power supply voltages appear in synchronism with the system clock signal. Generally, at least either a positive power supply voltage or a negative power supply voltage applied to the A/D converter is used in common with the digital signal processing circuit. Thus, the noise components are applied to the A/D converter and, as a result, the precision of the A/D conversion deteriorates. In some cases, the A/D converter malfunctions due to the noise components.
The noise components also arise due to the following reasons. A voltage drop across a parasitic resistance on the a power supply line is generated when the system clock signal changes its state. An unwanted radiation of a high-frequency noise component takes place when the system clock signal changes.
The above-mentioned problems are illustrated in FIG. 1. When a system clock CK1 rises at time t.sub.0, a noise component appears on a power supply voltage. Similarly, when the system clock CK1 falls, a noise component appears on the power supply voltage. In this way, noise components on the power supply voltage appear in synchronism with the rises and falls of the system clock signal CK1. An analog signal processing circuit, such as an A/D converter operates in synchronism with the system clock signal. Thus, when the system clock signal changes, the analog signal processing circuit is affected by the occurrence of noise components and, as a result, the precision of the A/D conversion deteriorates and a malfunction thereof is caused.